Triple Modular Redundancy with voting is a well known hardware technique used to mask failures in elements which are fully synchronous with each other. This invention provides means for achieving TMR operation where elements are asynchronous or skewed with respect to each other as is frequently expected to be the case as system clock rates continue to increase.
One current system claimed to incorporate asynchronous TMR is the recently announced Tandem S2 system. Although no detailed technical information is available, the Tandem approach is known to be significantly different from that described here. It uses a dual memory and the voting is upon memory reference.
This only known asynchronous TMR system is the Integrity S2 system recently announced by Tandem. That system is said to use dual global memories and to synchronize AND TO VOTE on all CPU accesses to global memory.
In the patented art, reference may be had to U.S. Pat. No. 4,907,232 to Harper et al. issued Mar. 6, 1990, which includes a plurality of processors and a network element connected to each of the processors. That patent mentions an application co-pending entitled "Synchronization of Fault-Tolerant Parallel Processing Systems, which was incorporated by reference, but not identified. This patent appears to relate to synchronizing the communication operations of the network, but does have a different system for providing voting messages. Other patents noted have been U.S. Pat. No. 4,667,284 which related to a Multiplexing control unit issued May 19, 1987; Fault Tolerant Output Circuits, to Smith et al issued Sep. 19, 1989; and van Gils of Holland issued U.S. Pat. No. 4,882,731 which disclosed the use of four data processing modules with related storage and local memory. Three substantially identical processors for parallel synchronous operations and a form of fault localizing was a subject discussed in U.S. Pat. No. 4,916,695 issued Apr. 10, 1990 to Bengt E. Ossfeldt of Sweden.
Asynchronous operation of TMR elements introduces a great many complexities. Consequently, it may not be desirable where synchronous TMR is feasible. There is, however, reason to believe that situations exist today where the improvements to such systems which will be described will be useful.